1. Field of the Invention
The present invention relates generally to valves having an expanding gate assembly, and more particularly but not by way of limitation, to an expanding gate valve which includes a valve closure member carrying a movable segment which moves relative to the valve closure member to effect sealed closure of the valve.
2. Brief Description of Prior Art
Valves with expanding gates have been taught in the prior art, wherein such valves comprise a gate having a female V-surface formed on the upstream side of the gate, a floating segment mounted on the gate with a complementary male V-surface disposed thereon to engage and coact with the female V-surface of the gate. The male/female V-surfaces constitute wedging means. This gate assembly is positioned between the seating surfaces on the valve body surrounding the fluid flow passageway to move transverse to the fluid flow path denoted by the fluid flow passageway.
The movement of the gate assembly transverse to the flow path causes the floating segment to contact a stop structure which arrests further movement of the segment. As the gate continues to move further, the segment is wedged outwardly away from the gate to sealingly engage the valve seating surface. This wedging movement of the segment is effected by the sliding action of the corresponding legs of the V-surfaces of the gate and the segment. When the gate assembly is moved in the opposite direction to open the valve, the segment is caused to collapse upon and nest with the gate in the complementary interfitting V-surfaces. In a nested configuration of the segment with the gate, the segment is fully retracted towards the gate and can move no further towards the gate after the wedging forces are relieved. This is the characteristic of a floating segment.
For smooth and easy operation, and to avoid damage to the valve, it is important that the wedging action of the gate-segment occur in only open or closed position of the valve, and that the segment remain in the collapsed condition when the gate is moving between the open and closed positions of the valve assembly. This prevents unnecessary dragging or binding of the gate assembly against the valve seats at a time when high integrity sealing is not essential. Pre-expansion of the segment during gate travel, referred to as back-wedging, can require excessive force on the stem to operate the valve, causing the valve assembly to jam, or cause failure of the stem.
The expanding gate valve is widely used in the Oil and Gas Industry for several decades. Fluid pressure as high as 20,000 psi is encountered in the deep well drilling area of the Oil & Gas Industry. However, even with its popularity in the Oil & Gas Industry for being able to handle high fluid pressure, the expanding gate valve has deficiencies. For one, it is an unidirectional valve in which the segment must be placed upstream only. The smooth operation of the valve is dependent on collapsing ability of the segment, which if placed downstream cannot collapse. Secondly, when the gate-segment assembly reciprocates between open and closed positions, the full force due to upstream fluid pressure acting on the segment is transferred to the downstream seat. As a result, the stem has to exert enough force to overcome the frictional resistance force between the gate and the downstream seat. To get an idea of the magnitude of this frictional force, consider a valve of 2 in. nominal dieter with fluid pressure of 10,000 psi. The fluid pressure exerts a force of over 15 tons on the downstream seat. With a coefficient of friction of 0.2 between the gate and the downstream seat, the 15 ton force translates into a stem force of 3 tons. With fluid pressure of 20,000 psi, figures are twice as large. The stem force is even larger if frictional resistance is encountered at the upstream seat. Thirdly, a gate valve is susceptible to a condition known in the trade or art as "pressure lock". Fluid pressure leakage past the valve seats can be trapped in the valve chamber with the result that the gate member is pressure locked against operating movement. To return the valve to normal operation, the valve chamber fluid pressure must be vented. Fourthly, sediments in the fluid can escape through the slit between the gate and the segment in the conduit portion of the gate assembly to the valve chamber, where it accumulates thereby creating an impediment to smooth operation of the valve. A frequent problem with prior art valve is that the gate segment assembly becomes jammed between the valve body seats. The segment expands in a direction parallel to the fluid flow path and in order to close the valve, the gate opened with ease when the gate/segment assembly is jammed between the valve body needs to exert tremendous wedging forces upon the valve body seats to counter the full force due to fluid pressure. The segment does not collapse and the valve cannot be seats.